1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method for transmitting feedback data in a multiple antenna system.
2. Related Art
Wireless communication systems are widely used to provide various types of communications. For example, voice and/or data are provided by the wireless communication systems. A conventional wireless communication system provides multiple users with one or more shared resources. For example, the wireless communication system can use various multiple access schemes such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA).
An orthogonal frequency division multiplexing (OFDM) scheme uses a plurality of orthogonal subcarriers. Further, the OFDM scheme uses an orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). A transmitter transmits data by performing IFFT. A receiver restores original data by performing FFT on a received signal. The transmitter uses IFFT to combine the plurality of subcarriers, and the receiver uses FFT to split the plurality of subcarriers. According to the OFDM scheme, complexity of the receiver can be reduced in a frequency selective fading environment of a broadband channel, and spectral efficiency can be improved through selective scheduling in a frequency domain by utilizing channel characteristics which are different from one subcarrier to another. An orthogonal frequency division multiple access (OFDMA) scheme is an OFDM-based multiple access scheme. According to the OFDMA scheme, a radio resource can be more efficiently used by allocating different subcarriers to multiple users.
Recently, to maximize performance and communication capability of the wireless communication system, attention is paid to a multiple input multiple output (MIMO) system. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas in order to improve efficiency of data transmission and reception. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then assembled into one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can increase with respect to a specific data transfer rate.
Hereinafter, downlink is defined as a communication link from a base station (BS) to a user equipment (UE), and uplink is defined as a communication link from the UE to the BS.
In general, the BS schedules uplink and downlink radio resources in the wireless communication system. User data or control signals are carried using the uplink and downlink radio resources. A channel for carrying user data is referred to as a data channel. A channel for carrying control information is referred to as a control channel.
For radio resource scheduling of the BS, the UE reports feedback data to the BS. In the multiple antenna system, the feedback data includes a channel quality indicator (CQI), a rank indicator (RI), a precoding matrix indicator (PMI), etc. The UE transmits the feedback data (e.g., CQI, RI, PMI, etc.) to the BS. According to the feedback data received from a plurality of UEs, the BS schedules uplink and downlink radio resources. A whole frequency band is divided into a plurality of subbands. The BS can schedule the radio resources for each subband. From an aspect of radio resource scheduling of the BS, it is most effective when the UE obtains respective CQIs and PMIs for all subbands and reports the obtained CQIs and PMIs to the BS. However, a significantly large overhead is caused when the CQIs and PMIs for all subbands are transmitted with limited radio resources.
Accordingly, there is a need for a method for effectively transmitting CQIs and PMIs in a multiple antenna system.